The present invention relates to an oblique incidence interferometer for measuring the shape of the surface of an object to be measured on the basis of interference fringes caused by light interference.
As an apparatus of this type, an oblique incidence interferometer is known which is used in the inspection of flatness of a silicon wafer in the process of manufacturing semiconductor devices. FIG. 2 is a diagram illustrating the configuration of an oblique incidence interferometer of a conventional example. Laser light emitted from an He-Ne laser 1 is focused at the focal point of a lens 2 to form a point light source, and after passing through a ground glass disk 3 disposed in its vicinity, the laser light is collimated into a parallel beam of light by a collimating lens 5 and is made incident upon a prism 6. The light reflected by a reference plane 7 of the prism 6 and the light transmitted through the reference plane 7 of the prism 6, reflected by a measurement surface 8 of the object to be measured, and transmitted again through the reference plane 7 of the prism 6 interfere with each other, and first-order interference fringes are formed on a screen 9. The interference fringes formed on the screen 9 are photographed by a camera 10, and their image is analyzed by an image processor 11, thereby measuring the shape of the measurement surface 8. It should be noted that the ground glass disk 3 is rotated at high speed by the motor 4 to eliminate the unevenness of the transmitted light due to the surface of the ground glass.
In such an oblique incidence interferometer, if the measurement surface 8 is close to the mirror surface or the interval between the measurement surface 8 and the reference plane 7 of the prism 6 is narrow, interference fringes caused by the light reflected by the reference plane 7 of the prism 6 and the light which was reflected and reciprocated more than twice between the measurement surface 8 and the reference plane 7 of the prism 6 can appear on the screen 9. These interference fringes are called higher-order interference fringes in contrast to the aforementioned first-order interference fringes. Unless these higher-order interference fringes are eliminated well, the result of measurement of the shape of the surface is apt to be erroneous.
To cope with this problem, in the above-described conventional example, the higher-order interference fringes are eliminated by making adjustment by moving the lens 2 in the direction of the optical axis of the projected laser light (in the direction of the optical axis of the lens 5) to offset the focal point of the lens 2 from the diffusing surface of the ground glass disk 3, so as to apparently enlarge the point light source and decrease the coherence of the laser light.
However, with the method in which the coherence is decreased by using the ground glass, since the transmittance at the ground surface is low, the power loss of the laser light results, so that the energy efficiency is poor. For this reason, there has been a drawback in that a laser light source having a large output is required, and the apparatus hence becomes expensive.
In view of the drawback of the above-described prior art, it is an object of the present invention to provide an apparatus which does not require a laser light source having a large output, and is capable of eliminating the higher-order interference fringes while keeping the first-order interference fringes necessary for the measurement.
The present invention provides the followings:
(1) An oblique incidence interferometer for measuring a shape of an measurement surface of an object, comprising:
a laser light source for emitting a laser light;
an oblique incidence optical system for making the laser light, emitted from the laser light source, incident obliquely upon the measuring surface through a reference plane; and
deflecting means for deflecting the laser light, which is to be made incident upon the measurement surface by the oblique incidence optical system, in a deflecting direction to decrease coherence of the laser light, the deflecting means continuously changing the deflecting direction;
wherein the shape of the measurement surface is measured based on interference fringes formed by reflected light reflected from the reference plane and the measurement surface.
(2) The oblique incidence interferometer according to (1), wherein the deflecting means includes:
a prism formed of transparent material; and
rotating means for continuously rotating the prism.
(3) The oblique incidence interferometer according to (2), further comprising:
adjusting means for adjusting a distance between the laser light source and the prism.
(4) The oblique incidence interferometer according to (2), further comprising:
imaging means for imaging the thus formed interference fringes, wherein the rotating means rotates the prism at a rotational speed depending on an image rate of the imaging means.
(5) The oblique incidence interferometer according to (1), wherein the laser light source includes a semiconductor laser light source.
(6) An oblique incidence interferometer comprising:
a laser light source disposed so as to make a laser beam incident obliquely upon a measurement surface of an object;
an optical element having a reference plane, the optical element being disposed between the laser light source and the measurement surface so that the reference plane is confronted with the measurement surface with a predetermined distance; and
a deflecting element, disposed between the laser light source and the optical element, for deflecting the laser light in a deflecting direction,
wherein the deflecting element continuously changes the deflecting direction.
(7) The oblique incidence interferometer according to (6), wherein the deflecting element includes a prism formed of transparent material.
(8) The oblique incidence interferometer according to (6), wherein the optical element includes a prism having the reference plane of a sufficient size relative to the measurement surface.
(9) The oblique incidence interferometer according to (6), further comprising:
a motor which continuously rotates the deflecting element.
(10) The oblique incidence interferometer according to (9), further comprising:
a screen and a camera which are disposed substantially symmetrically to the laser light source with respect to the measurement surface,
wherein the motor rotates the deflecting element at a rotational speed depending to an imaging rate of the camera.
(11) The oblique incidence interferometer according to (6), further comprising:
a moving mechanism which adjusts a distance between the laser light source and the deflecting element.
(12) The oblique incidence interferometer according to (6), wherein the laser light source includes a semiconductor laser light source.
The present disclosure relates to the subject matter contained in Japanese patent application No. Hei. 10-349651 (filed on Dec. 10, 1998), which is expressly incorporated herein by reference in its entirety.